Price is required,
because you want to maintain the price that was valid at the time of adding to the cart.
Should the price in the shop change in the meantime,
it will maintain this price.
If you would like to update the pages,
you have to do it before loading the cart page on your shop.

Same as relative cost, but it assumes that tax was included in the subtotal already, and only displays it (19% of subtotal value in example). Inclusive is a boolean value (0/1).
$cart->apply_cost(amount => 0.19, name => 'tax', label => 'Sales Tax', relative => 1, inclusive => 1);